1. Field of the Invention
The present invention relates to a built-in stator, a method of manufacturing the same, and a brushless direct-current ("BLDC") motor using such a stator. More particularly, it relates to a built-in stator having two coils wound in a star shape and operated in a 2-phase drive manner thereby assuring a simple structure and precluding torque ripple and loss of coil, and a BLDC motor employing the stator.
2. Description of the Related Art
Coreless-type BLDC motors may be classified into cylindrical core (radial) and coreless (axial) ones depending on whether a stator core exists.
The core-type BLDC motors are characterized as an internal magnet-type motor and an external magnet-type motor. The internal magnet-type motor includes a cylindrical coil-wound stator and a rotor of cylindrical permanent magnets provided to a plurality of protrusions formed on its inner circumference so as to be of electromagnetic construction. In the external magnet-type motor, a stator around which a coil is wound and a rotor having cylindrical permanent magnets are provided to a plurality of protrusions formed on its outer circumference.
Since its magnetic circuit has an axial-symmetric structure, the core BLDC motor makes little noise during operation and is suitable for low-speed rotation, creating desirable torque. This core BLDC motor, however, results in a waste of materials for making a stator and requires great expense for facility investment for mass production. In addition, since the core BLDC motor's stator and rotor are of complicated structure, it is not easy to make the motor compact, and it cannot assure high efficiency and creates undesirable torque.
A coreless BLDC motor was proposed in order to solve the above-described problems. Referring to the conventional coreless BLDC motor, rotors that each consist of an annular magnet and a yoke are fixed to a shaft, and stators around with a plurality of stator coils wound therearound are fixed to a casing. One end of the shaft is rotatably joined to the casing by means of a pair of bearings.
This coreless BLDC motor has a magnetic circuit axially created between the rotors consisting of a set of N-and-S pole magnets and the stators about which a plurality of the stator coils generating electromagnetic force are wound. Thus, even if a buffer spring is inserted between a pair of the bearings, the coreless BLDC motor generates great axial vibrations due to the stators' attracting or repelling force. Besides, the axial vibrations induce a resonance of the overall system employing the coreless BLDC motor during operation, thereby increasing the noise. Accordingly, the motor's efficiency is not decreased during high-speed rotation but gives rise to much noise.
In conclusion, the above-described coreless BLDC motor saves materials and has an advantageous yield aspect compared to the core BLDC motor. Moreover, it is possible to make it compact, which lowers the overall production costs and enhances its efficiency. The coreless BLDC motor, however, creates much noise due to axial vibrations during operation.
Korean Patent Application Nos. 96-976 and 96-977 filed on Jan. 18, 1996 by this applicant have proposed a double rotor/double stator-type coreless brushless direct-current (BLDC) motor which counteracts axial vibrations created during operation, and makes torque double or more.
The coreless-type blushless direct-current motor of a double rotor-type disclosed in Korean Patent Application No. 96-977 includes first and second disk-shaped rotors each having a plurality of north-polar magnets and south-polar magnets, said first and second rotors being disposed opposing to each other such that the corresponding magnets have an opposite polarity to each other; a rotating shaft connected to a central portion of the rotors through a bushing; left and right cases rotatably supporting opposite ends of the rotating shaft; and first and second stators each having a plurality of bobbinless stator coils for applying electromagnetic force to the first and second rotors in an opposite direction to each other, the first and second stators being mounted between the first and second rotors at a predetermined clearance.
In the above application a plurality of stator coils each wound around the middle of the first and second double rotors are of double-stator type structure, thus forming a magnetic circuit of symmetric structure with respect to stator and rotor shafts. The attracting force or repelling force of the same intensity is applied to the stator coils of the first and second stators in an opposite direction to each other, so the attracting force counteracts the repelling force thus minimizing an axial vibration acting on the first and second rotors.
The symmetric structure of the double/stator type coreless BLDC motor has a single body made by insert molding, and Korean Patent Application No. 96-18767 (filed on May 30, 1996) proposes this motor for enhancing the durability and reducing the production costs.
The BLDC motor, as shown in FIG. 1, includes upper and lower cases 71A and 71B defining a cylindrical case with a stator assembly 51 having an outer circumference 67 extending upward and downward and coupled between the upper and lower cases 71A and 71B.
Upper and lower rotors 73A and 73B each having a magnet dividing multi-polarity arrangement structure are fixedly coupled around the rotating shaft 77 through bushings 75A and 75B at upper and lower portions of the stator assembly 51.
The respective rotors 73A and 73B have eight magnets 81A and 81B. That is, four disk-type N-polar magnets and four disk-type S-polar magnets are alternately supported on a support 79 integrally formed with the bushings 75A and 75B and made of a polyethylene telephtalate or polybuthylene telephtalate, and, on its one side, annular-shaped magnetic yokes 83A and 83B are integrally attached, thereby forming a magnetic circuit with respect to the eight magnets 81.
The arrangement of the magnets 81A and 81B and the coils 55 of the stator assembly 51 is illustrated in FIG. 11. The oblique lined annular-shaped magnets 81A and 81B are disposed corresponding to the penetrating holes 65 of the coils 55.
An auxiliarly magnet 85 for detecting the location of the hole terminal is attached on the upper surface of the yoke 83A of the upper rotor 73A. The auxiliarly magnet 85 is disposed opposing to the hole terminal 89 of the printed circuit board 87 mounted on an inner circumference of the upper case 71A. An arm connector 91 to which the upper terminal 63A of the stator assembly 51 is press-coupled is mounted on one side of the control printed circuit board 87.
Upper and lower bearings 93A and 93B are fixed on central concave portions of the upper and lower cases 71A and 71B, respectively. The rotating shaft 77 of the rotors 73A and 73B is rotatably supported through the bearings 93A and 93B.
Reference numerals 95 and 97 denote a distance maintaining bushing and a screw for fixing the upper and lower cases 71A and 71B, respectively.
A bobbin-type stator assembly 51 comprises six bobbins 53 on which coils 55 are wound. These bobbins 53 are formed in an annular-shape by a resin insulating material with an auxiliarly printed circuit board 57.
Each central portion of the auxiliarly printed circuit board 87 and the stator body 59 is provided with a penetrating hole 61. On one side of the stator body 59, an upper terminal 63A for electrically connecting this stator assembly to a control printed circuit board 87 depicted in FIG. 10 and a lower terminal 63B for electrically connecting this stator assembly to another stator assembly when a multiple structure is adapted are formed.
Since each of the bobbin coils 55 is not wound by a separate winder and a plastic bobbin 53 is used, a single or multiple axial winder which is easy to automatize can be used to wind the bobbin coils 55, thereby minimizing the manufacturing cost by reducing the expense for facility investment.
In addition, the coils for the bobbin coil 55 can be selected from a normal insulating copper wire which is chipper than the bonding wire used for the bobbinless coil by 25% to 50%, reducing the expense for the coil.
The stator assembly 51 is a single body, and the bobbin 53 is used for winding the bobbin coil 55, thereby enhancing the productivity and lowering the production costs, and reliable insulation is provided between the coils 55, which is of damp-proof and rust-proof structure.
When integrally forming the stator assembly by insert molding using the bobbinless stator coil, instead of the bobbin coil, the current flowing along the coil and the magnetic flux density become two times the single stator structure, thereby decreasing an air gap and increasing an output of the motor as much as two times those of the single stator.
The conventional motor employs a plurality of fan-shaped coils L depicted in FIG. 6, and 6 or 9 stator coils (L1 to L6), 6 transistors (TR1 to TR6), and 3 location sensors (H1 to H3) are used in the 3-phase drive manner as shown in FIGS. 4A and 4B.
According to the 2-phase drive manner 4, 8 or 12 stator coils (L1 to L8), 4, 6 or 8 transistors TR1 to TR8, and 2 location sensors (H1 and H2) are used.
The above is shown in Table 1.
______________________________________ Number of Magnet Number Number of Number of Torque poles of Coils Transistors Sensors ripple ______________________________________ 3-phase 4, 8, 12 6, 9 6 3 Excellent 2-phase 6, 12, 18 4, 8, 12 4, 6, 8 2 Good ______________________________________
Referring to Table 1, the stator assembly is made by interconnecting terminals of 6 coils (L1 to L6), utilized (FIGS. 4 and 3) in the 3-phase drive manner, using an auxiliary printed circuit board (PCB), and 8 coils (L1 to L8) employed (FIG. 5) in the 2-phase drive manner. There is a need to solve the problems of a decrease of the productivity by preliminary connection of a plurality of coils and an increase of the production costs. The torque becomes high by increasing the number of the rotor's magnetic poles to obtain the maximum output of the motor of the same size, and simultaneously with this, the number of the coils must be increased.
In the conventional axial BLDC motor of FIG. 6 a conducting portion that produces the torque by the coil L acting with the magnet of the rotor (FIG. 2's 81A and 81B) during driving is parts LA and LC that are in parallel with a line radially stretching from the rotary shaft, and vertical intersections LB and LD are just necessary for maintenance of the coil L and indicative of a loss of the overall motor. The vertical intersections to which the torque of the rotor is not applied must be minimized in size. As a material of the coil a bonding wire that is 1.5 to 2 times as expensive as the ordinary insulating copper wire.
As shown in TABLE. 1, the 2-phase drive manner has a disadvantageous torque ripple aspect compared to the 3-phase drive manner, and a relatively large number of transistors. However, its drive circuit is economical and may be designed variously.